1. Field of the Invention
This invention relates to foam dielectric for coaxial cables. More particularly, the invention relates to an irradiated polyethylene (PE) foam dielectric with a chain extended characteristic, enabling cost efficient manufacture of coaxial cables with, for example, improved structural characteristics and operating temperature capabilities.
2. Description of Related Art
Coaxial cables may utilize a foam dielectric to support the inner conductor coaxially within the surrounding outer conductor. The foam dielectric of conventional coaxial cables may be comprised of, for example, a blend of high density polyethylene (HDPE) and low density polyethylene (LDPE). LDPE materials selected for this application typically have long chain branches which provide a stable foaming characteristic.
LDPE provides advantages of an improved foaming characteristic while the HDPE has a higher melting temperature as well as improved strength, crush resistance and attenuation characteristics. Conventional HDPE polymer, alone, has not typically been used as the foam dielectric because it does not normally have enough elongational viscosity to stabilize bubble growth during foaming. Because of the properties of each material, a foam dielectric is typically a blend of HDPE and LDPE materials.
A nucleant is typically added to the blend of HDPE and LDPE which is then subjected to a gas during the extrusion process to assist foaming. Conventional low density foams typically use either a single gas or a mixed gas foaming agent. The mixtures used contain an atmospheric gas in combination with a second agent such as butane, pentane or a refrigerant. It should be noted that the secondary gasses mentioned are objectionable because of flammability and/or environmental concerns.
A method used to improve the melting performance of the dielectric foam with minimal impact to dielectric properties subjects the dielectric foam to an electron beam to cross-link the polymer chains. However, the cross linked polymer chains take on a thermal set and cannot be melted again for reuse.
Competition in the coaxial cable market has focused attention on improving coaxial cable physical characteristics and electrical performance while minimizing overall costs, including materials costs. It is desirable from an environmental perspective to have a foam dielectric that can be melted again for reuse and to minimize the use of environmentally objectionable secondary gasses.
Therefore, it is an object of the invention to provide a coaxial cable and method of manufacture that improves upon the prior art.